The specific goal is to test the hypotheses that messenger RNA expression profiles (mRNA profiles) and microRNA expression profiles (miRNA profiles) of urinary cells/peripheral blood cells/ renal allograft biopsies are predictive and diagnostic of acute rejection, subclinical acute rejection, and chronic allograft nephropathy (CAN) in renal allograft recipients randomized to either a low dose or a standard dose tacrolimus regimen. Specific Aim 1: To evaluate the prognostic utility of urinary cell/peripheral blood cell/allograft biopsy mRNA/miRNA profiles, measured at the time of randomization, for predicting the subsequent development of (i) clinical acute rejection; (ii) subclinical acute rejection and (iii) CAN. Specific Aim 2: To determine whether renal allograft recipients randomized to a low tacrolimus regimen exhibit a different longitudinal pattern of mRNA/miRNA profiles in urinary cells/peripheral blood cells compared to recipients randomized to a standard tacrolimus regimen. Specific Aim 3: To evaluate the diagnostic utility of urinary cell/peripheral blood cell/ allograft biopsy mRNA/miRNA profiles for the diagnosis of subclinical acute rejection or CAN. The study cohort for the proposed study will be 160 renal allograft recipients enrolled in our single center randomized controlled trial (RCT) comparing a low tacrolimus regimen (target trough level: 3.0 to 5.0ng/ml) with a standard tacrolimus regimen (6.0-8.0ng/ml). Randomization will occur at 3 months post-transplantation and the subjects will undergo protocol renal allograft biopsy at the time of randomization and at 12 and 33 months post-randomization. The RCT is supported by the combination of a NIH- Clinical and translational award to Weill Cornell, industry, and institutional funds and no funds are requested for the performance of RCT in this application. This mRNA/miRNA profiling study leverages the RCT and the proposed research may lead to the development of noninvasive and mechanistically informative molecular biomarkers for the safe minimization of immunosuppressive therapy in organ graft recipients.